Heat treating method



Feb. 13, 1934. E. F, NORTHRUP 1,946,875

HEAT TREATING IETHOD Filed Aug. 7, 192

\ TEMPERATURE Patented Petra- 934 HEAT TREATING METHOD Edwin FitchNorthrup, Princeton, N. J., assignor to Ajax Electrothermic Corporation,Trenton, I N. J., a corporation of New Jersey Application August 7,1928. Serial No. 298,101 '11 Claims- (01.148-) My invention relates toheating systems for rolls, intended to vbe tempered.

The main purpose of the invention lies in determining the heat gradientof a mass to be-tempered by the rate of inductive heat input to themass.

A further purpose is to heat a cylindrical charge largely at the surfaceand at a rate of speed of heating which shall prevent undue conductivetravel of the heat into the interior of the roll but which will permitsuflicient travel of the heat into the interior of the roll to form aprogressively reducing temperature in preparation of a cushion backingto graduate hardness to support the outer roll surface.

A further purpose is to provide an inductor coil long enough to takeobjects of maximum length and to provide taps both for power and powerfactor correction whereby intermediate sections of the length may beheated.-

A further purpose is to provide different currents about the main partof the top of a roll or other object than about the ends of the roll orother body whereby the roll or other tempered body is tempered to agreater degree in the major part of its length than it is immediately atthe ends, reducing the danger of cracking of the ends of the roll.

A further purpose is to heat the greater part of a length of a roll byan inductor carrying the applied current and the current of a tunedcircuit and to heat the immediate ends of the roll to be tempered bymeans of the current in the tuned circuit only.

My invention relates both to the furnace and to methods which may becarried out in inductor furnaces.

Further purposes will appear in the specification and in the claims.

Figure 1 is a broken diagrammatic elevation illustrating one applicationof my invention.

Fig. la is a modification of the arrangement shown in-Fig. 1.

Figure 2 is a diagrammatic view comparing curves of heat gradient.

Figures 3 and 4 are diagrammatic sectional elevations illustratingapplication of another part of my invention.

In the drawing similar numerals indicate like parts.

Steel rolls for rolling mill purposes afford good illustrations ofcharges which require maximum hardness at the surface with progressivelydecreasing hardness toward the axis. Rolls differ in length and diameterof the parts to be tempered. As the rolls form a familiar and generallydistributed'example which will well serve to explain these twoapplications the invention will be applied to rolls in the furtherexplanation.

This hardness gradient must be secured by a temperature gradient in theroll when it is quenched, giving the roll a maximum temperature at thesurface and a progressively reduced temperature within the roll toward aminimum at the axis.

Experiments indicate. that the temperature half-way between the surfaceand the axis should be somewhere in the neighborhood of 300 F. lowerthan the temperature at the surface.

In the application of alternating current inductively to heat the rollthe depth of penetration of the current within the body of the roll,following Steinmetz formula (Transient Phenomena Chap. VII, par. 63, Eq.40) will be directly proportional to the square root of the resistivityof the metal and inversely proportional tothe square root of thefrequency of the current in the inductor.

While the depth of penetration is here a technical term the extent ofpenetration of induced energy within the roll is also directlyproportional to the square root of the resistivity and inverselyproportional to the square root of the frequency. 1

It is thus possible to determine the depth at which the heat will begenerated in the body of the roll by selecting the frequency and tosecure generation of heat at different distances in from the surfaceaccording to the frequency selected.

Other conditions affect the selection er the frequency whichis'preferably determined by that frequency at which increasing cost ofgenerators due to the increase of frequency and reducing cost of powerfactor correction condensers because of the increase of frequencydetermine a minimum cost for the entire equipment.

I have discovered that the temperature gradient can be controlled verynicely for anyassumed frequency within very widelimits by adjustment ofthe speed of power input, increasing or reducing the power input so thatthere will be just enough inward travel of heat units by thermalconductivity to give the heat gradient which is desired.

With a selected number of ampere turns in the inductor the frequencyaffects both the rate of heat input and the distance from the surface atwhich heat is generated.

For any intended classor size of charge the no frequency can be modifiedfrom that indicated for maximum economy to a higher frequency where itis desirable to generate the heat in the charge nearer to the surface ofthe charge or to a lower frequency where it is desirable to generate theheat at a greater average depth beneath the surface of the charge.

Assuming, therefore, that the frequency has been set for a giveninstallation, either upon the basis of economy or upon a variation fromit to secure technical efficiency, the current supplied can be adjustedso as to enough not only to heat the outside shell of the roll beforethere has been any considerable travel of heat inwardly by conduction toheat the interior of the roll, but to secure any temperature dropdesired at a given distance from the surface.

The steel used for different rolls will have nearly the same resistivityand nearly the same rate of exterior heat loss, by conduction,convection and radiation in all rolls or other objects to be tempered,in any given installation. The frequency will be selected in advance andwill ordinarily be the same for all work of approximately the samediameter and class. We may, therefore, assume that the two factors ofresistivity and frequency are fixed and that the temperature gradient isdependent upon the extent to which the heat is conducted inwardly towardthe axis of the roll while the power is on.

If we could consider the heat as developed in the exterior surface ofthe roll instantaneously, the adjoining parts of the body of the rollwould remain at normal temperature. On the other hand, if with a smallpower input the roll be heated slowly, the heat developed near thesurface of the roll will travel inwardly toward the axis of the rolluntil the roll is highly heated throughout, giving a very flat curve ofheat gradient, which will be nearly a straight horizontal 1 line.

, chain '7, so that after heating, the roll can be The temperaturegradient must, therefore, be determined primarily by the speed withwhich the roll is heated. I have discovered that the temperaturegradient curve can be passed through any point desired, representing anintended drop in temperature at a given distance from the surface, bythe simple expedient of adjusting the time element of the heating,speeding up the input and thus the heating if the curve would otherwisepass above the point, to permit less heat conduction to the interior ofthe roll and slowing the input and hence the heating if the curve wouldotherwise pass below the point.

The external heat loss to the atmosphere by conduction, convection andradiation is so small as compared with the heat conduction within themetal toward the axis as to be substantially negligible.

In Figure 1 I have shown a roll 5 held in position by any suitable clamp6 and a supporting lowered quickly by gravity or dropped into atempering pool 8 beneath it.

The roll is inductively heated by an alternating current from any source9 passed through coil 10. Both the source and coil are shownconventionally as in power-factor correction by condensers 11, whoseconnection may be greatly varied.

In Figure 2 I have shown a circle 12 intended to represent the outercircumference of the roll within and upon which I have plotted threecurves.

Distances within the roll are plotted along radius 0 P, temperatures atthe axis appear upon give a power input high,

radius 0 Q, and temperatures at the circumference are plotted upontangent P R. Time appears in the steepness of the curves, a slower rateof heat input being evident in greater heat conduction toward the centerof the roll and consequent flattening of the curves by which the heatgradients are plotted.

The axis of the roll is shown at O and normal temperature at thecircumference is shown at P. All the curves start at the same initialheight P R, representing the required temperature at the surface of theroll. Let us assume that the conditions for maximum strength andtoughness of the metal adjoining the hardened surface require a givendrop of temperature represented by a drop from R to S, at a distance infrom the surface represented by P T. Then the gradient curve must passthrough the point where the horizontal line S U cuts the vertical line TU. The curve b is the only one of the three which passes through thepoint U.

The fact that the curve 1: falls below the point U indicates that thespeed of heating the roll has been too high to allow proper heatconduction toward the interior. The rate of heat input resulting incurve 0, which lies above point U, has evidently been too slow and hasallowed too much conduction of heat within the roll.

It will be noted that the speed of heating in curve a has been so greatthat the roll has not been affected at the axis a, that the slow heatingrepresented by the curve 0, has allowed the roll to become quite hot atthe center, as seen at c, and that the intermediate speed of heatingshown in curve b has heated the roll somewhat at the axis as seen at b,butnot nearly to the extent seen in curve 0.

Experiments with thermo-couple determination of the actual temperatureat the desired distance within the roll have shown that the requireddrop in temperature can be secured very exactly by adjustment of thespeed at which the heat is developed in the roll.

The variation in size and character of work for which the heat gradientcurve sought will be true within any limits of error permitted may alsobe found experimentally for each different type of work to be handledwithin this variation or with such change for difference in diameter,

,for example as the tests have shown to be desirable; the rate of heatinput may then be maintained for successive pieces of work withoutfurther test. Partly for this reason the connections in Figure 1 havenot been shown as adjustable. However one reason for changing the inputappear in Figures 3 and 4.

When the roll has been raised to the required temperature it may beplunged quickly into the quenching pool by gravity. In the illustrationit may be lowered quickly or dropped directly through the coil.

In order to indicate that it is not essential to my broad invention tohave the condenser power factor correction across the coil in parallel Ihave shown connections in Figure 1a in which the condenser is in serieswith the coil. If this form be used for tapping into partial coillengths the capacity should be adjusted.

It is desirable and in many cases necessary that the apparatus forcarrying out the invention be capable of supplying variant power inputin order that it may be suited to charges differing in inputrequirements and may not berestricted to use upon one general type ofproduct only. There is a further reason for providing an adjustment ofpower input in that even with rolls there is wide variation in thelengths of the surfaces to be tempered, for example, from a roll havinga full length rolling surface, such as is seen in Figure 1, to onehaving a short length of rolling surface only as in Figure 3 at 5'. Myinvention provides for power input adjustment which takes care of theseseveral conditions.

I illustrate a coil long enough to receive a roll having maximum lengthof operating roll face and provide it with tap connections by which thepower input may be connected to any portion of the coil, correspondinggenerally in the case of a short roll surface with the position of theroll surface upon that particular roll, and with condenser power factorcorrection capable of adjustment in position and also in amount, adaptedto be applied to a portion of the coil corresponding approximately withthat of the roll surface which is to be heated.

In the form shown in Figures taps 13 are provided at intervals, and aresupplied with terminals 14 which may be engaged by arms 15, 16 connectedwith the generator and 17, 18 connected with the condensers. Theconnections shown are for a short roll surface.

In Figure 4 the construction is intended to be substantially the same asin Figure 3, but the tap connections provided are those for a fulllength roll.

In both figures the power input is tapped across a shorter portion ofthe coil length than is the condenser power factor correction.

Where the portion of the charge to be tem pered extends for a short partonly of the length of the roll, connections may be made by the taps soas to include those coils only which surround this portion of thecharge, whatever its position lengthwise of the coil. Preferably theselection is made so that the power input is extended over a smallernumber of turnsof the coil than the condenser taps with the purpose andeffect that the voltage upon the condensers is higher than the voltageof the-generator; When the furnaces are placed in parallel surging ofthe condensers is prevented, or is usefully employed to carry thesurging current through those turns of the inductor lying betweenthecondenser taps and the power input taps.

I tune the circuit including the inductor turns and the condensers atleast approximately and thus get a very much larger current through theinductor turns than the generator current.

In Figures 1, 3 and 4 where the current is applied across a length ofinductors shorter than the length of the part to be tempered and thepower factor correction is applied across a length of inductorcorresponding substantially with the length .of the surface to betempered, it will be evident that the tuned circuit traverses the entireinductor but that the applied current traverses the shorter length only.As a result of this the greater part of the length of the roll beingtempered will be heated by the two currents which have a combinedheating effect. However, the extreme ends of the roll will be heated bythe tuned circuit only. This makes it possible to temper the body of theroll which is to perform a of the length of the charge which is intendedto engage the metal during the rolling operation.

It will be further evident that my invention secures maximum hardnesswith maximum strength and toughness by supporting the outer surface upona backing which progressively increases in toughness and strength; andthat not only can the results of one use of the 'invention be exactlyduplicated at another time butthat variation of results can be securedintelligently and surely, slightly increasing the speed of input tosteepen the heat gradient curve or slightly reducing the speed toflatten the curve.

In view of my invention and disclosure variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the art, to obtain all or part of the benefits of myinvention without copying the structure shown, and I, therefore, claimall such in so far as they fall within the reasonable spirit and scopeof my invention.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. The method of controlling the heat gradient preparatory to temperinga charge so as to give maximum hardness at a surface with maximumstrength in the adjoining supporting body structure, which consists indelivering the energy into the me'al of the charge adjacent its surfaceat 105 a high rate of heat input in order to heat the surface rapidly inselecting the time element of heating with respect to conduction of heatwithin the body 0 allow the heat to enter to a predetermined distanceand in quenching the charge l when the heat generated at and near thesurface has had time by conduction, normally to travel tothe'predetermined distance.

2. The method of controlling-the heat in a charge to be tempered so asto get a desired heat gradient in the charge and. to avoid excessivelyheating the supporting body of the charge, which consists in heating theexterior of the charge at high speed, by inducing a flow of current init close to its outer surface, the flow of current being greater aboutthe body of the charge than about the ends of the charge and inquenchingthe charge quickly before the heat has had time to travel excessively byconduction into the body of the charge.

3. In the art of tempering, the method of heating charges of variantaxial length of surface to a high temperature without excessive heatingof the adjacent interior of the body of the charge, which consists incirculating a primary current about the surface of the charge to behardened, within limits axially shorter than the portion of the chargeto be tempered, tuningthe circuit by correcting for power factorthroughout substantially the entire axial length of said portion andquenching the charge afterits exterior surface has been heated andbefore the heat has communicated farby conduction within the body of thecharge.

4. The method of adjusting the temperature gradient within a metalliccharge to be tempered, which consists in establishing a temperature lossfor a given distance from the surface, in electromagnetically developingthe heat in the outer part of the body of the charge and in applying theheat input at such a speed as, with the travel of heat within the chargeby conductivity, will secure the required loss of temperature within thedistance indicated.

5. The method of securing a desired heat drop in temperature within acharge for tempering the charge within an electric furnace coil, whichconsists in applying the heat input at such a rate that the differencebetween the heat put into the charge and that conducted toward thecenter of the charge gives the required difference in temperature, and'in changing the heat input for a given charge as required to securethis gradient by change in the number of coils spanned by condenserpower factor correction.

6. The method of heating cylindrical solid charges of variant axiallength and position of surface to be heated within an inductor, whichconsists in connectingan alternating current power input to energize theinductor opposite the central portion of the charge to be heated and inproviding power factor correction across a greater axial extent ofinductor than that spanned by the power input for a given chargeconnection and including it, to induce a larger current flow in thecentral part of the length of a charge than in its ends.

'7. The method of applying alternating current power input having apower factor correction across a larger number of turns than the powerinput to a coil adapted to receive charges to be tempered to differentaxial lengths of the portion to be tempered and in variant positionswithin the coil, which consisL-s in shifting both the power input andthe power factor correction to that part of the inductor opposite theportion of the charge to be tempered and substantially corresponding inlength with said part to be tempered.

8. .The method of heating a solid by electromagnetic induction to temperit, which consists in passing current from a source of current supplyabout a portion of the body of the charge, between the ends of thecharge in forming a tuned circuit therefrom and in passing the currentof the tuned circuit about a greater length of the charge than thatencompassed by the current supply whereby the body of the charge is morefully heated than the ends of the charge.

9. The method of heating a charge by electromagnetic induction in it andquenching to temper it. which consists in inducing a lesserelectromagnetic current in the ends of the part to be tempered than inthe intermediate part of the charge ind quenching the charge while thetemperatures due thereto are unequal.

10. The method of controlling the heat conditions of a charge beneathits surface preparatory to tempering it so asv to give a maximumhardness at its surface with maximum strength in the adjoiningsupporting body structure, which consists in electrically inducingalternating current in the outer part of the charge at a high rate ofinput, determining the range of travel of the induced current within theouter surface by the selection of the frequency, determining the extentof input spread of heat from the range of the charge in which current isinduced by the time of application of the current before quenching,establishing a definite temperature loss at an interior point anddetermining the temperature at the interior point and the gradientbetweeen the surface temperature and the temperature at the interiorpoint by the relation of the frequency, the input and the time ofapplication of the current.

11. The process of heating steel articles as a step in the operation ofzone-hardening, which comprises generating the major portion of the heatwithin an outer zone of metal by a high frequency electric inducedcurrent adapted to heat said zone to a hardening temperature, regulatingthe speed of the heating operation by the rate of the power input, andregulating the depth of penetration of the hardening heat by thefrequency of the current and the rate of power input.

EDWIN FITCH NORTHRUP.

